Method for making retroreflective articles having polymer multilayer reflective coatings

ABSTRACT

A method for making retroreflective articles that have a layer of optical elements and a multilayer reflective coating disposed on the optical elements. The reflective coating reflects light back into the optical elements so that it can be returned toward the light source. The multilayer reflective coating has at least one polymer layer made by condensing a curing a pre-polymer vapor.

This application is a divisional of U.S. patent application Ser. No.09/259,100, filed Feb. 26, 1999, U.S. Pat. No. 6,172,810, incorporatedherein by reference.

The present invention pertains to methods for making retroreflectivearticles that have a multilayer reflective coating that includes one ormore polymer layers disposed in optical association with a layer ofoptical elements.

BACKGROUND

Retroreflective articles have the ability to redirect incident lightback towards the light source. This unique ability has led to thewide-spread use of retroreflective articles on various substrates. Forexample, retroreflective articles can be used on flat inflexiblesubstrates, such as road signs and barricades; on irregular surfaces,such as corrugated metal truck trailers, license plates, and trafficbarriers; and on flexible substrates, such as road worker safety vests,a jogger's shoes, roll up signs, and canvas-sided trucks.

There are two major types of retroreflective articles: beaded articlesand cube-corner articles. Beaded articles commonly use a multitude ofglass or ceramic microspheres to retroreflect incident light. Typically,the microspheres are partially embedded in a support film, and aspecular reflecting material is provided between the layer ofmicrospheres and the support film. The reflecting material can be ametal layer (for example, an aluminum coating as disclosed in U.S. Pat.Nos. 3,700,478 and 4,648,932) or an inorganic dielectric mirror made upof multiple layers of inorganic materials that have different refractiveindices (as disclosed in U.S. Pat. Nos. 3,700,305 and 4,763,985).Categories of beaded articles include exposed lens, enclosed lens, andencapsulated lens types. Exposed lens beaded articles have a layer ofmicrospheres that are exposed to the environment. Enclosed lens beadedarticles have a protective layer such as a transparent polymer resincontacting and surrounding the front side of the microspheres.Encapsulated lens articles have an air gap surrounding the front side ofthe microspheres and have a transparent film hermetically sealed to asupport film to protect the microspheres from water, dirt, or otherenvironmental elements.

In lieu of microspheres, cube-corner sheeting typically employs amultitude of cube-corner elements to retroreflect incident light. Thecube-corner elements project from the back surface of a body layer. Inthis configuration, incident light enters the sheet at a front surface,passes through the body layer to be internally reflected by the faces ofthe cube-corner elements, and subsequently exits the front surface to bereturned towards the light source. Reflection at the cube-corner facescan occur by total internal reflection when the cube-corner elements areencased in a lower refractive index media (e.g., air) or by reflectionoff a specular reflective coating such as a vapor deposited aluminumfilm. Illustrative examples of cube-corner sheeting are disclosed inU.S. Pat. Nos. 3,712,706; 4,025,159; 4,202,600; 4,243,618; 4,349,598;4,576,850; 4,588,258; 4,775,219; and 4,895,428.

SUMMARY OF THE INVENTION

The present invention provides a new approach to supplyingretroreflective articles with reflective coatings. In brief summary, thepresent invention provides a retroreflective article that comprises: (a)a layer of optical elements; and (b) a reflective coating that isdisposed in optical association with the optical elements, thereflective coating comprising a plurality of layers wherein (i) at leasttwo adjacent layers have different refractive indices, and (ii) thereflective coating includes multiple polymer layers that each have anaverage thickness that is less than about 10% of an average size of theoptical elements.

Retroreflective articles of this invention differ from knownretroreflective articles in that the optical elements have an associatedreflective coating that comprises multiple polymer layers. The polymerlayers can have indices of refraction and thicknesses selected such thatthe overall multilayer reflective coating reflects light in a desiredwavelength range. Known retroreflective articles have used metalreflective layers, which in some instances can be subject to oxidationfrom air or moisture. When oxidized, the reflective layer can suffer asubstantial loss in its reflective ability. Retroreflective articleshave also employed multilayered inorganic dielectric mirrors that can besusceptible to air or moisture induced corrosion that can degradereflectivity and/or lead to delamination of the layers. The polymermultilayer reflective coating of the present invention is beneficial inthat it can be made highly reflective to light in a desired wavelengthband(s), while also being capable of resisting undesirable environmentaleffects, such as air and/or moisture induced corrosion, to which knowninorganic reflective coatings can be susceptible. The multilayerreflective coating of the present invention can also include inorganicand/or non-polymer layers disposed adjacent to or between the multiplepolymer layers, for example, to help overcome limitations of knowninorganic reflector coatings by rendering them more resistant to water,acids, bases, corrosion or other environmental degradation.

The above and other advantages of the invention are more fully shown anddescribed in the drawings and detailed description of this invention. Itis to be understood, however, that the description and drawings are forillustrative purposes and should not be read in a manner that wouldunduly limit the scope of the invention.

GLOSSARY

As used in this document, the following terms have the followingdefinitions:

“Index of refraction” or “refractive index” is a material property thatrepresents the ratio of the phase velocity of an electromagnetic wave ina vacuum to that in the material.

“Optical association” means the reflective coating is positionedrelative to the optical elements such that a significant portion oflight transmitted through the optical elements can strike the reflectivecoating and be reflected back into the optical elements.

“Optical elements” are light transmissive elements capable of alteringthe direction of light that enters the elements so that at least aportion of the light can ultimately be returned towards the lightsource. The “size” of an optical element refers to its characteristicwidth, depth, height, or length.

“Polymer layer” refers to a layer of material that includes organicmolecules that have multiple carbon-containing monomer units that arelinked in regular or irregular arrangements.

“Reflective coating” refers to a coating that is capable of reflectingincident light and that is made up of one or more layers of material.

“Retroreflective” means having the characteristic that obliquelyincident incoming light is reflected in a direction antiparallel to theincident direction, or nearly so, such that an observer or detector ator near the light source can detect the reflected light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view schematic representation of a portion of thebackside of a cube-corner retroreflective article 10 in accordance withthe present invention.

FIG. 2 is a cross-sectional representation of the cube-cornerretroreflective article 10 shown in FIG. 1 taken along line 2—2.

FIG. 3 is an enlarged inverted view of a portion of a cube-cornerelement 16 taken from region 3 of FIG. 2.

FIG. 4 is a cross-sectional schematic of a portion of a beadedretroreflective article 40 in accordance with the present invention.

FIG. 5 is an enlarged view of a portion of a microsphere element 30taken from region 5 of FIG. 4.

FIG. 6 is a schematic representation of adjacent layers in a multilayerreflective coating 34 useful in the present invention.

FIG. 7 is a schematic representation of a coating method useful in thepresent invention.

DETAILED DESCRIPTION

FIG. 1 shows a portion of a retroreflective article 10 that has aplurality of optical elements, which in this embodiment are shown ascube-corner elements 16, each defined by three faces 18 arranged to forma pyramidal shape. The cube-corner optical elements 16 are arranged asan ordered array and are shown to protrude out of the page of thedrawing. The cube-corner elements 16 are disposed as matched pairs in anarray on one side of the sheeting. Each cube-corner element 16 has theshape of a trihedral prism that has three exposed planar faces 18. Theplanar faces 18 may be substantially perpendicular to one another (as inthe corner of a room) with the apex 20 of the prism vertically alignedwith the center of the base. The angle between the faces 18 typically isthe same for each cube-corner element in the array and is about 90°. Theangle, however, can deviate from 90° as is well-known; see, for example,U.S. Pat. No. 4,775,219. Although the apex 20 of each cube-cornerelement 16 may be vertically aligned with the center of the base of thecube-corner element—see, for example, U.S. Pat. No. 3,684,348—the apexalso may be canted to the center of the base as disclosed in U.S. Pat.No. 4,588,258. Thus, the present invention is not limited to anyparticular cube-corner geometry; however, of the many known cube-cornerconfigurations (see, for example, U.S. Pat. Nos. 4,938,563; 4,775,219;4,243,618; 4,202,600; and 3,712,706), the cube-corner sheeting describedin U.S. Pat. No. 4,588,258 may be preferred because it provides wideangle retroreflection among multiple viewing planes.

FIG. 2 shows a cross-sectional representation of the retroreflectivearticle 10 taken along line 2—2 of FIG. 1. Retroreflective article 10has a body portion 12 from which the cube-corner elements 16 protrude.The body portion 12 has a front side 13 through which incident light Ienters. A reflective coating 14 is disposed on the article 10 in opticalassociation with the cube-corner elements 16. Incident light I reflectsoff cube-corner faces 18 and becomes redirected in the general directionof the incident beam, as indicated by reflected light beam R. Areflective coating 14 may increase the efficiency of reflections off thecube-corner faces 18 in some instances.

The body portion 12 and the optical elements 16 may be made fromessentially any suitable light transmissible material. Preferably, thebody portion and cube-corner elements comprise light transmissiblepolymers. This means that the polymer will allow light, particularlyactinic radiation or visible light, to pass therethrough. Preferably thepolymer is able to transmit at least 70 percent of the intensity of thelight incident upon it at a given wavelength. More preferably, thepolymers that are used in the retroreflective sheeting of the inventionhave a light transmissibility of greater than 80 percent, and morepreferably greater than 90 percent.

FIG. 3 shows a magnified view of the portion of the cube-corner elementindicated by circle 3 in FIG. 2. Reflective coating 14 includes multiplepolymer layers. For purposes of illustration, FIG. 3 shows a reflectivecoating 14 made up of six layers arranged as alternating layers of twodifferent materials, at least one of which is a polymer, the materialshaving different refractive indices n₁ and n₂. Although six alternatinglayers of two different materials are shown in FIG. 3, the reflectivecoating can include two or more layers, and any suitable combination oftwo or more polymer layers. Preferably, the reflective coating has 2 to200 layers, and more preferably 2 to 50 layers. For conformance to theprofile of the cube-corner optical elements, it is preferred that eachindividual layer be thin relative to the cube-corner element heights(cube-corner heights measured from base to apex). The individual layersin the multilayer coating have thicknesses of less than about 10% of thecube-corner element height, more preferably less than about 5% of thecube-corner element height. In addition, the layers should havethicknesses that are appropriate for reflection of light in a desiredwavelength range. The selection of layer thickness and refractive indexof the materials in the multilayer reflective coating is discussed inmore detail below.

FIG. 4 illustrates a beaded retroreflective article 40 that includesoptical elements in the form of microspheres 30 that are partiallyembedded in a binder layer 32. A reflective coating 34 is disposedbetween the layer of microspheres 30 and the binder layer 32. Optionalsubstrate layer 36 can be used to add structural support. The beadedretroreflective article 40 as configured in FIG. 4 is typically referredto as an “exposed lens” beaded retroreflective article. An “exposedlens” sheeting is one where the optical elements, in this casemicrospheres, are exposed to the ambient environment, namely air.Optionally, a protective layer (not shown) that covers or encapsulatesthe exposed portions of the microspheres can also be provided to make“enclosed lens” or “encapsulated lens” beaded retroreflective sheeting.Examples of exposed lens sheetings are described in the following U.S.Pat. Nos.: 5,812,317; 4,763,985; and 3,700,478. Examples of encapsulatedlens products are shown in U.S. Patent Nos. 5,784,198; 5,066,098; and4,896,943. As shown in FIG. 4, incident light I that enters amicrosphere can be refracted toward the center of the microsphere,reflected off the reflective coating 34 behind the microsphere, andredirected out of the microsphere in the general direction of theincident light, as indicated by reflected light beam R.

The microspheres used in a beaded product of the invention preferablyare substantially spherical in shape to provide uniform and efficientretroreflection. The microspheres preferably also are highly transparentto minimize light absorption so that a large percentage of incidentlight is retroreflected. The microspheres often are substantiallycolorless but may be tinted or colored in some other fashion. Themicrospheres may be made from glass, a non-vitreous ceramic composition,or a synthetic resin. In general, glass and ceramic microspheres arepreferred because they tend to be harder and more durable thanmicrospheres made from synthetic resins. Examples of microspheres thatmay be useful in this invention are disclosed in the following U.S. Pat.Nos.: 1,175,224; 2,461,011; 2,726,161; 2,842,446; 2,853,393; 2,870,030;2,939,797; 2,965,921; 2,992,122; 3,468,681; 3,946,130; 4,192,576;4,367,919; 4,564,556; 4,758,469; 4,772,511; and 4,931,414.

The microspheres typically have an average diameter of about 10 to 500μm, and preferably of about 20 to 250 μm. Microspheres smaller thanthese ranges tend to provide lower levels of retroreflection, andmicrospheres larger than these ranges may impart an undesirably roughtexture to the retroreflective article or may undesirably reduce itsflexibility when flexibility is a desired property. Microspheres used inthe present invention typically have a refractive index of about 1.2 to3.0, preferably about 1.6 to 2.7, and more preferably about 1.7 to 2.5.

FIG. 5 shows a magnified view of a portion of the microsphere element 30indicated by region 5 in FIG. 4. Reflective coating 34 has multiplepolymer layers, which in this instance is made up of six layers arrangedas alternating layers of two different materials, at least one of whichis a polymer, the layers having different refractive indices n₁ and n₂.As in the cube-corner retroreflected article described above, the sixalternating layers of two different materials as shown in FIG. 5 aremerely illustrative. In general, a multiple layer reflective coatingthat has two or more layers representing two or more differentrefractive indices can be used. As discussed above, the reflectivecoating preferably has 2 to 200 layers, and more preferably 2 to 50layers. Basically, what has been said above regarding the reflectivecoating 14 in the cube-corner retroreflective article 10 is likewiseapplicable to reflective coating 34, and vice versa. For goodconformance to the profile of the microspheres, it is preferred thateach layer be thin relative to the microsphere diameters. The individuallayers in the multilayer coating have thicknesses of less than about 10%of the microsphere diameters, more preferably less than about 5% of themicrosphere diameters.

Without reference to specific types of optical elements, the individualpolymer layers of the reflective coating typically have thicknesses thatare less than 10% of the average size of the optical elements of theretroreflective article. Preferably, the individual polymer layers havethicknesses that are less than 5% the average size of the opticalelements. Without regard to the dimensions of the optical elements, thepolymer layers preferably have thicknesses of less than 3 μm, morepreferably less than 2 μm, and even more preferably less than 1 μm.

Preferably, each layer of the reflective coating is clear or essentiallycolorless to minimize light absorption and maximize light reflection,however, a great variety of visual effects may be achieved, if desired,when one or more of the layers are colored, such as with a dye. Suchcoloring agent, if provided, preferably leaves the reflective coatingsubstantially transparent.

As mentioned above, the individual layers of the multilayer reflectivecoating disposed on retroreflective articles according to the presentinvention preferably have thicknesses that are appropriate forreflection of light in a desired wavelength range. In general andaccording to known optics, light having wavelengths within a desiredwavelength range can be reflected when the combined optical thickness oftwo adjacent layers that have different indices of refraction is an oddmultiple of one-half of a wavelength in the desired wavelength range.FIG. 6 indicates the relationship between layer thickness, index ofrefraction, and angle of incidence for an arbitrary incident light rayI. For light incident perpendicular to the surface of the layers (normalincidence), the combined optical thickness of the adjacent layers issimply n₁t₁+n₂t₂, where n is the index of refraction, t is thethickness, and the subscript denotes the layer. For light incident at anangle θ measured from a line perpendicular to the surface of the layers,a more general approximation of the combined optical thickness ofadjacent layers can be given by (n₁t₁+n₂t₂)/cosθ. This approximationimproves for small θ, and is best for θ less than about 20°.

The difference in refractive index between adjacent layers can affectthe reflectivity of the multilayer reflective coating. In general, thelarger the difference between n₁ and n₂, the stronger the reflectionfrom the pair of layers. Preferably, in the multilayer reflectivecoating of the present invention, adjacent layers have indices ofrefraction that differ by at least 0.02, and more preferably by at least0.05 or more, and still more preferably by at least 0.1 or more. Due tomaterials considerations, the difference in refractive index foradjacent polymer layers is typically less than about 1.2, and moretypically less than 1, although higher refractive index differencesmight be achieved, and are contemplated for use in this invention,depending on the materials used.

Generally, higher refractive indices can be obtained using non-polymermaterials, such as certain metallic, inorganic, organometallic, andceramic materials, than can be obtained for polymeric materials. Forexample, materials with a relatively high refractive index for visiblelight include PbO (index of 2.61), SiC (index of 2.68), TiO₂ (index of2.71), and PbS (index of 3.91). These values can be compared withtypical polymeric materials with refractive indices that range fromabout 1.3 to 1.7. Thus, refractive index differences of more than 1.2,or even more than 2, can be obtained in some instances when non-polymerlayers are placed adjacent to polymer layers in the reflective coating.Examples of non-polymer inorganic and inorganic dielectric materialsthat may be used include: high index materials such as CdS, CeO₂, CsI,GeAs, Ge, InAs, InP, InSb, ZrO₂, Bi₂O₃, ZnSe, ZnS, WO₃, PbS, PbSe, PbTe,RbI, Si, Ta₂O₅, Te, and TiO₂; and low index materials such as Al₂O₃,AlF₃, CaF₂, CeF₂, LiF, MgF₂, Na₃AlF₆, ThOF₂, and SiO₂.

The number of layers in the multilayer reflective coating can alsoaffect reflectivity. More layers can generally improve reflectivity,although two or more layers are suitable for use in the presentinvention. In general, as the average refractive index differencebetween adjacent layers is increased, fewer layers can be used toachieve similar results. The number and thickness of layers can alsoaffect the coloration of the reflection from the multilayer reflectivecoating. For example, when more than two layers are used, the opticalthickness of some layers can be varied relative to the optical thicknessof other layers. By varying optical thicknesses in the layers of thereflective coating, different pairs of adjacent layers can be made toreflect light in different wavelength bands so that an overall broaderrange of wavelengths can be reflected by the reflective coating as awhole. For applications where it is desirable to reflect most of thelight in the visible spectrum (that is, light having wavelength of about380 nanometers (nm) to about 750 nm), the optical thickness of adjacentlayers can be varied so that overlapping wavelength bands can bereflected to substantially cover a desired portion of the visiblespectrum.

In other embodiments, a particular coloration of reflected light mightbe desirable, and in that case the optical thickness of adjacent layersthat have different indices of refraction can be selected tosubstantially reflect light in a desired wavelength band (or bands) andto substantially transmit light outside of the desired wavelength band(or bands). In these applications, a more intense reflection of light ina desired wavelength band (and a better transmission of light outsidethe desired wavelength band) can typically be obtained by using morelayers in the multilayer reflective coating, preferably 5 or morelayers, more preferably 10 or more layers.

Retroreflective articles that have multilayer reflective coatingsaccording to the present invention that selectively reflect light ofcertain wavelengths or wavelength bands can be used to retroreflectdesired wavelengths uniformly over the entire article as well as toretroreflect different wavelengths or wavelength bands from differentareas of the article. For example, the distribution of layer thicknessesand indices of refraction in a reflective coating on one portion of aretroreflective article can be made different from the distribution oflayer thicknesses and indices of refraction in a reflective coating onanother portion of the same retroreflective article. In this way, thelight reflected from different areas of the retroreflective article canhave a different coloration or intensity. This can be useful, forexample, when the areas of different coloration or intensity formgraphic images, letters, words, characters, or other indicia. The terms“coloration” and “color” have been used here for convenience and candenote selected wavelengths of invisible light (i.e., infraredradiation, ultraviolet radiation, and so on) as well as visible light.

A variety of layer patterns can be used to form multilayer reflectivecoatings on retroreflective articles according to the present invention.For example, FIGS. 3 and 5 show multilayer reflective coatings made upof alternating layers of two different materials, thereby forming apattern (i.e., A,B,A,B, . . . ). Other layer patterns can also be used,including those involving three-component systems (e.g., A,B,C,A,B,C . .. , A,B,C,B,A,B,C,B, . . . , and others), other multi-component systems,as well as systems where no overall pattern exists. Layer variationsinclude index of refraction variations (i.e., variations in materials)as well as thickness variations to achieve the desired arrangement ofcombined optical thickness among adjacent layers. In addition, asindicated above, optional inorganic and/or non-polymer layers can beincluded in the multilayer reflective coating, for example adjacent toor between multiple polymer layers. These optional layers can includemetals, metal oxides, inorganic dielectrics (such as various oxides,nitrides, sulfides, and others), ceramic materials, organometallics, andother such non-polymer materials. Such individual layers are generallycapable of transmitting light on their own, but when combined with otherlayers of different refractive indices, allow a coating to be producedwhich as a whole is capable of reflecting light. Generally, anycombination of such thin multiple layers which includes at least twopolymer layers and which allows light to be reflected is contemplated bythe invention. Examples of other suitable layers are described in U.S.Pat. Nos. 4,763,985 and 3,700,305.

The polymer layers used in the reflective coating can be disposed inoptical association with optical elements of retroreflective articlesusing methods now known or later developed which are suitable fordisposing multiple layers of polymeric materials that have desiredthicknesses and indices of refraction. Such methods can includesolvent-borne coating methods, liquid reactive coating methods,extrusion coating methods, gravure coating methods, physical andchemical vapor deposition methods, plasma deposition methods, filmlamination methods, and the like. In general, these methods involvecoating each layer in a sequential fashion. Some methods, however, arealso amenable to simultaneous disposition of multiple layer stacks. Forexample, multiple polymer layers can be coextruded as a multiple layerstack onto retroreflective articles. Alternatively, pre-formed polymermultilayer films can be laminated to retroreflective articles, forexample by using heat and/or pressure to conform a multilayer polymerfilm to the optical elements of the retroreflective article.

Multilayer reflective coatings can be provided in optical associationwith the optical elements of retroreflective articles in a substantiallycontinuous fashion across the entire retroreflective area of theretroreflective articles. Alternatively, multilayer reflective coatingscan be formed in a discontinuous fashion to optically associate one ormore multilayer coatings with one or more selected portions of the layerof optical elements. This can be done, for example, by layer depositionthrough a mask and/or subsequent removal of the coating material fromundesired portions. See, for example, International Publication WO95/31739 (corresponding to U.S. patent application Ser. No. 09/140,083).

Exemplary methods of coating multiple polymer layers include thepre-polymer vapor deposition methods taught in co-filed and co-pendingU.S. patent application Ser. No. 09/259,487 (entitled “Method of CoatingMicrostructured Substrates with Polymeric Layer(s), AllowingPreservation of Surface Feature Profile”), the disclosure of which iswholly incorporated by reference into this document. Briefly, thesemethods involve condensing a pre-polymer vapor onto a structuredsubstrate, and curing the material on the substrate. These methods canbe used to form polymer coatings that have controlled chemicalcomposition and that preserve the underlying profile of the structuredsubstrate. Multiple coatings of the same or different material can beapplied in this fashion to form multiple polymer layers in a multilayerreflective coating. This method provides the capability to form uniformcoatings of desired thickness in optical association with the opticalelements of retroreflective articles using a wide range of materials.

Preferred methods of making multilayer polymer coatings in opticalassociation with the optical elements of retroreflective articles caninclude aspects of the coating process shown in FIG. 7. The process canbe performed at atmospheric pressure, optionally enclosing the coatingregion in a chamber 118 (e.g., for providing a clean environment, forproviding an inert atmosphere, or for other such reasons), or at reducedpressure where chamber 118 is a vacuum chamber. Coating material 100,supplied in the form of a liquid monomer or pre-polymer, can be meteredinto evaporator 102 via pump 104. As described in detail below, thecoating material 100 can be evaporated by one of several techniques,including flash evaporation and carrier gas collision vaporization.Preferably, the coating material can be atomized into fine dropletsthrough optional nozzle 122, the droplets being subsequently vaporizedinside evaporator 102. Optionally, a carrier gas 106 can be used toatomize the coating material and direct the droplets through nozzle 122into evaporator 102. Vaporization of the liquid coating material, ordroplets of the liquid coating material, can be performed via contactwith the heated walls of the evaporator 102, contact by the optionalcarrier gas 106 (optionally heated by heater 108), or contact with someother heated surface. Any suitable operation for vaporizing the liquidcoating material is contemplated for use in this invention.

After vaporization, the coating material 100 can be directed through acoating die 110 and onto the optical elements 111 of retroreflectivearticle 112. A mask (not shown) can optionally be placed between thecoating die 110 and the retroreflective article 112 to coat selectedportions of the optical elements 111. Optionally, the surfaces of theoptical elements 111 can be pretreated using an electrical dischargesource 120, such as a glow discharge source, silent discharge source,corona discharge source, or the like. The pretreatment step isoptionally performed to modify the surface chemistry, for example, toimprove adhesion of coating material to the retroreflective article, orfor other such purposes. In addition, the surfaces of the opticalelements 111 can optionally be pretreated with an adhesion promoter, asdiscussed below.

Retroreflective article 112 is preferably maintained at a temperature ator below the condensation temperature of the monomer or pre-polymervapor exiting the coating die 110. Retroreflective article 112 can beplaced on, or otherwise disposed in temporary relation to, the surfaceof drum 114. The drum 114 allows the retroreflective article 112 to bemoved past the coating die 110 at a selected rate to control the layerthickness. The drum 114 can also be maintained at a suitable biastemperature to maintain the retroreflective article 112 at or below thepre-polymer vapor's condensation temperature.

After being applied on the optical elements 111, the coating materialcan be solidified. For coating materials containing radiation-curable orheat-curable monomers, a curing source 116 can be provided downstream tothe coating die 110 in the drum rotation direction (indicated by arrow124). Any suitable curing source is contemplated by this invention,including electron beam sources, ultraviolet lamps, electrical dischargesources, heat lamps, and the like.

A reflective coating that has two or more different polymer layers canbe disposed in optical association with the optical elements 111 of aretroreflective article 112 by supplying at least a second coatingmaterial (not shown). After condensing the first coating material on theoptical elements 111, a second coating material can be condensed on apreviously deposited layer or layers, preferably after the previouslydeposited layer or layers have been cured. Addtional coating materialscan be deposited as desired. Optionally, inorganic, organometallic,and/or non-polymer layers can also be deposited using suitable methods,now known or later developed, including sputtering, chemical vapordeposition, electroplating, condensing from a solvent, and other suchmethods. These optional layers may be deposited directly on the opticalelements before the polymer layers are formed, after the polymer layersare formed, or between polymer layers.

A particularly preferred optional layer is an adhesion promoter coatedbetween the optical elements of the retroreflective article and thepolymer layers of the multilayer reflective coating. Adhesion promoterscan be selected to improve adhesion between the multilayer reflectivecoating and the optical elements. For example, a silane coupling agentcan be used that promote adhesion between polymer layers of themultilayer reflective coatings of the present invention and opticalelements which can be, for example, glass or ceramic microspheres,molded polycarbonate cube-corner elements, or other such opticalelements. Exemplary silane coupling agents includeaminopropyltriethoxysilane, glycidoxypropyltrimethoxysilane,methacryloxypropyltrimethoxysilane, and vinyltrimethoxysilane. Inaddition, titanate coupling agents can be used as adhesion promoters,examples of which include isopropyl tri(dioctyl)phosphato titanate,dimethacryl oxoethylene titanate, and titanium(tetraisopropoxide).Silazanes such as hexamethyldisilazane can also be used as adhesionpromoters. Examples of silane coupling agents are disclosed in U.S. Pat.No. 5,200,262 to Li.

Apparatuses suitable for carrying out various aspects of the methodillustrated in FIG. 7 are described in co-filed and co-pending U.S.patent application Ser. No. 09/259,487 (entitled “Method of coatingMicrostructured Substrates with Polymeric Layer(s), AllowingPreservation of Surface Feature Profile”), in U.S. Pat. Nos. 6,012,647and 6,045,864, and in U.S. Pat. Nos. 4,722,515; 4,842,893; 4,954,371;5,097,800; and 5,395,644. In particular, an apparatus that may besuitable for carrying out certain aspects of the method illustrated inFIG. 7 under vacuum conditions is commercially available on acustom-built basis from Delta V Technologies, Inc, Tucson, Ariz.Apparatuses and portions of apparatuses that may be suitable forcarrying out these and other aspects of the method illustrated in FIG. 7are described in more detail in the cited documents.

Exemplary monomers and oligomers suitable for use in the process shownin FIG. 7 include acrylates, methacrylates, acrylamides,methacrylamides, vinyl ethers, maleates, cinnamates, styrenes, olefins,vinyls, epoxides, silanes, melamines, hydroxy functional monomers, andamino functional monomers. Suitable monomers and oligomers can have morethan one reactive group, and these reactive groups may be of differentchemistries on the same molecule. Pre-polymers can be mixed to achieve abroad range of optical properties such as index of refraction in thelayers of the reflective coating. It can also be useful to coat reactivematerials from the vapor phase onto a substrate already havingchemically reactive species on its surface, examples of such reactivespecies being monomers, oligomers, initiators, catalysts, water, orreactive groups such as hydroxy, carboxylic acid, isocyanate, acrylate,methacrylate, vinyl, epoxy, silyl, styryl, amino, melamines, andaldehydes. These reactions can be initiated thermally or by radiationcuring, with initiators and catalysts as appropriate to the chemistryor, in some cases, without initiators or catalysts. When more than onepre-polymer starting material is used, the constituents may be vaporizedand deposited together, or they can be vaporized from separateevaporation sources.

The deposited pre-polymer materials can be applied in a substantiallyuniform, substantially continuous fashion, or they can be applied in adiscontinuous manner, for example, as islands that cover only a selectedportion or portions of the optical elements. Discontinuous applicationscan be provided in the form of characters, numerals, or other indicia byusing, for example, a mask or other suitable techniques, includingsubsequent removal of undesired portions.

Pre-polymer vapor deposition is particularly useful for forming thinfilms having a thickness of about 0.01 micrometers (μm) to about 50 μm.Thicker layers can be formed by increasing the exposure time of thesubstrate to the vapor, by increasing the flow rate of the fluidcomposition to the atomizer, or by exposing the substrate to the coatingmaterial over multiple passes. Increasing the exposure time of theretroreflective article to the vapor can be achieved by adding multiplevapor sources to the system or by decreasing the speed at which thearticle travels through the system. Layered coatings of differentmaterials can be formed by sequential coating depositions using adifferent coating material with each deposition, or by simultaneouslydepositing materials from different sources displaced from each otheralong the substrate travel path.

After condensing the material on the article, the liquid monomer orpre-polymer layer can be cured. Curing the material generally involvesirradiating the material on the substrate using visible light,ultraviolet radiation, electron beam radiation, ion radiation and/orfree radicals (as from a plasma), or heat or any other suitabletechnique. When the article is mounted on a rotatable drum, theradiation source preferably is located downstream from the monomer orpre-polymer vapor source so that the coating material can becontinuously applied and cured on the surface. Multiple revolutions ofthe substrate then continuously deposit and cure monomer vapor ontolayers that were deposited and cured during previous revolutions. Thisinvention also contemplates that curing occur simultaneously withcondensing, for example, when the optical elements have a material thatinduces a curing reaction as the liquid monomer or pre-polymer materialcontacts the surface. Thus, although described as separate steps,condensing and curing can occur together, temporally or physically.

Table I lists a few examples of polymer and pre-polymer materials thatcan be disposed in optical association with the optical elements ofretroreflective articles using various methods. The known refractiveindex of the monomer and/or the polymer made from the monomer is givenfor each material. Different refractive indices can be achieved bychoosing these or other starting materials that either have a desiredrefractive index or that can be mixed with one or more other materialsto obtain a desired refractive index.

TABLE I Refractive Refractive Polymer or Supplier of index indexpre-polymer material monomer (monomer) (polymer) Poly(vinyl naphthalene)Aldrich — 1.6818 (Milwaukee, WI) Poly(styrene) Aldrich 1.547 1.592Poly(lauryl methacrylate) Aldrich 1.445 1.474 Poly(trimethyl cyclohexylAldrich 1.456 1.485 methacrylate) Poly(pentafluoro-styrene) Aldrich1.406 — Poly(trifluoroethyl Aldrich 1.361 1.437 methacrylate)Poly(dibromopropene) Aldrich 1.5573 — Poly(benzyl methacrylate) Aldrich1.512 1.568 Poly(ethylene glycol phenyl Aldrich 1.518 — ether acrylate)Poly(pentadecafluoro-octyl 3M 1.328 1.339 acrylate) (St. Paul, MN)Poly(ortho-sec-butyl 3M 1.562 1.594 dibromophenyl acrylate) Ethoxylatedtrimethylol- Sartomer 1.4695 — propane triacrylate (Exton, PA)Tris(2-hydroxy ethyl) Sartomer 1.4489 — isocyanurate triacrylateEthoxylated Bisphenol A Sartomer 1.4933 — diacrylate 1,6 hexanedioldiacrylate Sartomer 1.456 — Isooctyl acrylate Sartomer 1.4346 —Isobornyl acrylate Sartomer 1.4738 — Tripropylene glycol Sartomer 1.44 —diacrylate

Other polymers that may be suitable are disclosed in co-filed andco-pending U.S. patent application Ser. No. 09/259,487 (entitled “Methodof Coating Microstructured Substrates with Polymeric Layer(s), AllowingPreservation of Surface Feature Profile”).

EXAMPLE

Advantages and objects of this invention are further illustrated in theExample set forth hereafter. It is to be understood, however, that whilethe Example serves this purpose, the particular ingredients and amountsused and other conditions recited in the Example are not to be construedin a manner that would unduly limit the scope of this invention. TheExample selected for disclosure is merely illustrative of how to makevarious embodiments of the invention and how the embodiments generallyperform.

Glass microspheres that had an average diameter of 40 to 90 μm and thathad a refractive index of 1.93 were partially embedded into a temporarycarrier sheet, forming a substrate referred to as a beadcoat carrier.The beadcoat carrier was taped onto the chilled steel drum of a monomervapor coating apparatus such as described in U.S. Pat. No. 4,842,893.The apparatus used a flash evaporation process to create a pre-polymervapor that was coated using a vapor coating die. The vapor coating diedirected the coating material onto the beadcoat carrier. The beadcoatcarrier was mounted such that rotation of the drum exposed the embeddedmicrospheres to, in order, a plasma treater, the vapor coating die, andan electron beam curing head. The deposition took place in a vacuumchamber.

Alternating layers of sec-butyl(dibromophenyl acrylate) (SBBPA), asdescribed in International Publication WO 98/50805 (corresponding toU.S. patent application Ser. No. 08/853,998), and tripropylene glycoldiacrylate (TRPGDA) were evaporated and condensed onto the beadcoatcarrier while the chilled steel drum was maintained at −30° C. The SBBPAmonomer had an index of refraction of about 1.56 and the TRPGDA monomerhad an index of refraction of about 1.44. The drum rotated to move thesample past the plasma treater, vapor coating die, and electron beamcuring head at a speed of 38 meters per minute (m/min). A nitrogen gasflow of 570 milliliters per minute (ml/min) was applied to the 2000 Wattplasma treater. The room temperature TRPGDA liquid flow was 1.2 ml/min,and the heated SBBPA liquid flow was 1.1 ml/min. The monomer evaporatorwas maintained at 295° C., and the vapor coating die was 285° C. Thevacuum chamber pressure was 2.2×10⁻⁴ Torr. The electron beam curing gunused an accelerating voltage of 7.5 kV and 6 milliamps current. Thealternating layers were applied by opening the SBBPA monomer flow valveat the monomer pump for one drum revolution then closing the SBBPAmonomer flow valve and simultaneously opening the TRPGDA monomer flowvalve for the next revolution.

This procedure was repeated for 60 alternating layers, each layer beingcured before the next layer was deposited. The beadcoat carrier coatedwith the 60 alternating layers was then coated with about 0.7millimeters (mm) of a rapid-curing, general purpose epoxy adhesive assold by ITW Devcon, Danvers, Mass., under the trade designationPOLYSTRATE 5-MINUTE EPOXY. The epoxy was allowed to cure at ambientconditions for 1 hour before stripping away the beadcoat carrier to givea retroreflective article that had a layer of glass microspheres and amultilayer reflective coating comprising 60 alternating polymer layersdisposed behind the microspheres.

As a comparative example, glass microspheres were embedded into abeadcoat carrier and were coated with about 0.7 mm of the same epoxywithout vapor depositing polymer layers onto the microspheres. Thecarrier sheet was stripped away after curing the epoxy for 1 hour. Theretroreflectance of the Example and the comparative example wereevaluated by measuring the percentage of incident light that wasretroreflected by the samples. The measurements were performed as afunction of wavelength for light in the visible spectrum (wavelengths of400 nm to 800 nm). The retroreflectance from the Example that had themultilayer reflective coating was about a 2.5% to 3.5% throughout therange of wavelengths whereas the comparative sample without themultilayer reflective coating had about a 1.5% reflectance throughoutthe range. This indicated that the multilayer polymer coating acted as areflector and improved the retroreflectivity relative to the comparativeexample.

All of the patents and patent applications cited are incorporated intothis document in total as if reproduced in full.

This invention may be suitably practiced in the absence of any elementnot specifically described in this document.

Various modifications and alterations of this invention will be apparentto one skilled in the art from the description herein without departingfrom the scope and spirit of this invention. Accordingly, the inventionis to be defined by the limitations in the claims and any equivalentsthereto.

What is claimed is:
 1. A method for making a retroreflective articlethat comprises the steps of: providing a layer of optical elements; andforming a multiple layer reflective coating in optical association withthe layer of optical elements by (i) coating a first layer having afirst index of refraction over the layer of optical elements and (ii)coating a second layer over the first layer, the second layer having asecond index of refraction different from the first index of refraction,wherein at least one of the first and second layers is formed bycondensing and curing a pre-polymer vapor.
 2. The method of claim 1,wherein more than one layer of the multiple layer reflective coating isformed by condensing and curing a pre-polymer vapor.
 3. The method ofclaim 2, wherein at least two different pre-polymer vapors are used toform at least two different layers of the reflective coating.
 4. Themethod of claim 1, wherein at least one layer of the multiple layerreflective coating is formed by depositing a non-polymer layer that is ametal oxide layer, an inorganic dielectric layer, an organometalliclayer, or a ceramic layer.
 5. The method of claim 1, further comprisingthe step of vaporizing a liquid composition containing a monomer or anoligomer to form the pre-polymer vapor.
 6. The method of claim 5,wherein the step of vaporizing a liquid composition comprises flashevaporation.
 7. The method of claim 5, wherein the step of vaporizing aliquid composition comprises atomizing the liquid composition intodroplets and vaporizing the droplets.
 8. The method of claim 1, whereincuring the pre-polymer vapor comprises exposing the condensedpre-polymer vapor to radiation.
 9. The method of claim 1, wherein curingthe pre-polymer vapor comprises heating the condensed pre-polymer. 10.The method of claim 1, wherein curing the pre-polymer vapor occurssimultaneously with condensing.
 11. The method of claim 1, wherein thepre-polymer vapor includes one or more of acrylates, methacrylates,acrylamides, methacrylamides, vinyl ethers, maleates, cinnamates,styrenes, olefins, vinyls, epoxides, silanes, melamines, hydroxyfunctional monomers, or amino functional monomers.
 12. The method ofclaim 1, wherein the layer of optical elements includes microspheres.13. The method of claim 1, wherein the layer of optical elementsincludes cube-corner elements.